Single-photon avalanche diodes (SPAD), also called Geiger-mode avalanche photodiodes, are suitable to detect extremely weak radiation. Avalanche photodiodes having the pn junction biased in the reverse direction become conducting when the applied voltage exceeds a breakdown voltage. A single-photon avalanche diode is operated with reverse bias voltage above the breakdown voltage, in the so-called Geiger-mode. In this mode of operation the electric field strength in the space charge region can attain values of more than 500 kV/cm, so that already few electron-hole pairs generated by incident photons and injected into the depletion layer can trigger a self-sustaining avalanche current of several mA.
US 2012/0025340 A1 discloses a vertical silicon photomultiplier for Geiger-mode operation. A pattern of trench electrodes provides anodes and cathodes of a pixel array. A pn-junction layer is arranged at the walls of the cathodes.
US 2010/0148040 A1 discloses a single-photon avalanche diode comprising an n-type semiconductor body providing an active region including the cathode, a trench extending through the body around the active region and providing a laterally isolating region, a depletion region, and a p-type anode region at a surface of the active region.
U.S. Pat. No. 7,462,889 B2 discloses an avalanche photodiode with a ring shape trench region surrounding a light receiving region.
US 2008/0191240 A1 discloses an avalanche photodiode with an outer trench provided in the periphery of a light absorption layer surrounding the depletion region to interrupt the path of the dark current.
US 2007/0145411 A1 discloses a trench polysilicon diode.